1. Field of Invention
The present invention generally relates to an electric resistance welding apparatus and method of manufacture, and more particularly to a welding apparatus having a transformer of improved construction.
2. Discussion of Prior Art
Resistance welding machines of the type concerned typically employ a frame for fixedly supporting the laminated magnetic circuit of a transformer having two primary winding coils and a secondary circuit connected to the welding electrodes of the machine. Because the welding electrodes are movably mounted to permit a workpiece to be clamped therebetween and the secondary circuit is fixed relative to the primary by the frame, each electrode is connected to the secondary circuit by a secondary shunt circuit made of flexible braids. These braids of the shunt circuit are subject to considerable fatigue during repeated welding operations and must be periodically replaced. Additionally, this secondary shunt circuit has a length which detrimentally adds to the overall impedance of the transformer.
In French Pat. No. 69 16 670, filed Jan. 22, 1969, the Applicant of the present invention proposed an electric welding apparatus of improved construction which reduces the impedance contribution of the secondary circuit by employing a transformer having a secondary circuit which is movable, with the welding electrodes, relative to a primary circuit adapted for connection to an alternating current source. The secondary winding of the secondary circuit is formed from a pair of parallel rods movably mounted inside of the transformer and electrically interconnected at one end by a flexible braid. The free end of each rod is connected to a welding electrode in such a fashion so as to form, together with the workpiece to be welded, the secondary circuit of the transformer.
This type of transformer construction, in a welding apparatus, offers a large number of advantages in that the secondary circuit is in direct contact with the workpiece thus reducing the impedance of the secondary circuit and providing direct welding. The secondary circuit is also reduced to its simplest form, being limited to the movable rods, which do not belong to the magnetic circuit of the transformer, and the corresponding portion of the workpiece which is clamped between the electrodes during welding. Moreover, the external secondary circuit has a constant impedance over any given time period and since this secondary circuit is in direct contact with the workpiece, the transformer can be constructed with smaller dimensions and a correspondingly lower power requirement, thus providing a savings in energy resulting in a reduction in the cost of operation.
While the transformer described above provides many advantages, the design is not particularly adapted for use with automatic welding apparatus and industrial robot manipulators where engineering efforts are directed to reducing the complexity, weight and overall size of the welding equipment.
Furthermore, in the construction of the transformer described by the above-noted French Patent, the secondary circuit is formed by two separate parallel rods each guided individually inside of the transformer and interconnected by means of a flexible shunt. As a result, the transformer must be dismantled for maintenance and a relatively complicated guiding structure must be provided for each individual rod, thus increasing the cost, complexity and overall weight of the welding apparatus.